Retinitis pigmentosa (RP) is a leading form of inherited blindness in humans. Of the three general modes of inheritance (autosomal dominant, autosomal recessive, and X-linked), X-linked RP (XLRP) is associated with a severe form of disease involving both rod and cone photoreceptors as primary targets (Berson 1993; Sandberg and others 2007). Over 70% of X-linked RP and 10%-20% of all RP cases are caused by mutations in the gene encoding RPGR (Bader and others 2003; Branham and others 2012; Churchill and others; Pelletier and others 2007). Given that mutations in well over 100 genes are currently known to cause RP and the greater severity of X-linked disease, RPGR is one of the most important RP disease genes.